Allow cascade downloads for APPROVED actions

[akobrin1]

This change updates cascade download validation to accept both DONE and APPROVED action states.
It also adds coverage to ensure APPROVED is accepted and invalid states are rejected.
Changes:

• download.go: accept ActionStateApproved; improve error message.
• stream_send_error_test.go: add tests for APPROVED and invalid (PENDING) states.

[roomote]

Rooviewer   See task on Roo Cloud

Review complete. No issues found.

The changes correctly extend cascade download validation to accept both DONE and APPROVED action states. The implementation is clean and the test coverage is appropriate.

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